The ice storm that devastated Maine 20 years ago was caused by an unusual mix of weather conditions, but experts say that, though a repeat is unlikely, it could happen again.

Ice storms aren’t unusual New England, but their effects usually are not as widespread or as long-lasting as they were during the epic one that struck in early 1998, according to John Cannon, senior forecaster with the National Weather Service in Gray.

“What made the storm so extraordinary was the longevity of it,” Cannon said. “The conditions fed themselves.”

Tom Hawley, another NWS forecaster in Gray, said that Maine gets minor ice storms every winter—a thin coat of ice accumulates on trees and power lines for a few hours before melting away. But the storm of January 1998, in which thick ice encased much of the state, was an extremely rare event that might not be repeated in 100 years.

“I doubt I’ll see that again in my lifetime,” Hawley said.

The initial weather system that brought freezing rain first to Quebec and northern New York, then east to Maine during the first week in January in 1998 was expected to clear quickly, said Cannon, who has worked in Maine for the weather service since before the storm. However, successive waves of low atmospheric pressure kept up a steady, freezing drizzle for several days. Fog and low clouds hovered over much of interior Maine, aggravating the buildup of ice on trees and power lines.

In a normal ice storm, ice builds up to more than a quarter inch. But in January 1998, the stationary front that kept warm air aloft and colder air at ground level lingered over Maine for nearly a week. In much of interior Maine, a layer of ice as much as 3 inches thick in places built up on anything that wasn’t heated, making travel extremely hazardous and downing trees, utility poles and even transmission line towers. Thousands of Mainers were without power for weeks.

“Most [ice storms] are fairly brief,” Cannon said. “That particular storm shut down the state. Not too many of them can do that.”

No one knows when Maine will experience another fierce ice storm, but global warming is not expected to make such storms less likely, according to Sean Birkel, a professor at University of Maine and the state’s official climatologist.

The world’s oceans naturally go through warming and cooling periods, the best-known example of which occurs in the tropical Pacific. The phenomenon known as El Nino, in which surface waters are warmer than average, irregularly alternates every two to seven years with a cooling period known as La Nina. The Pacific is now in a La Nina phase.

In 1997 and 1998, the warmer-than-average El Nino effect was particularly strong, Birkel said. Around the same time, waters in the northern Atlantic Ocean also experienced periodic warming, which suggests that ice storms like the one in 1998 are not incompatible with the warming of the oceans that is caused by climate change, he said.